Bicycles have had gears for almost a century, and they allow to tackle hilly areas easily. Also, the Netherlands is notoriously windy, and a headwind is just as difficult as a hill.
No, what makes the Netherlands different is their street design prioritizing safety rather than speed at all costs. When the streets feel safe from speeding drivers, more people choose to ride a bike.
Not at all. I simply suspect that you are uninformed about why cycling is popular in the Netherlands. In the 60s the Netherlands was just as flat as it is today, but it wasn't a cycling paradise. It all changed with the campaign "Stop de Kindermoord" (literally translated as "Stop the Child Murder"), which began in 1972.
No, what makes the Netherlands different is their street design prioritizing safety rather than speed at all costs. When the streets feel safe from speeding drivers, more people choose to ride a bike.